What is a VLAN?

VLANs (Virtual LANs) are logical grouping of devices in the same broadcast domain. VLANs are usually configured on switches by placing some interfaces into one broadcast domain and some interfaces into another. Each VLAN acts as a subgroup of the switch ports in an Ethernet LAN.

VLANs can spread across multiple switches, with each VLAN being treated as its own subnet or broadcast domain. This means that frames broadcasted onto the network will be switched only between the ports within the same VLAN.

A VLAN acts like a physical LAN, but it allows hosts to be grouped together in the same broadcast domain even if they are not connected to the same switch. Here are the main reasons why VLANs are used:

  • VLANs increase the number of broadcast domains while decreasing their size.
  • VLANs reduce security risks by reducing the number of hosts that receive copies of frames that the switches flood.
  • you can keep hosts that hold sensitive data on a separate VLAN to improve security.
  • you can create more flexible network designs that group users by department instead of by physical location.
  • network changes are achieved with ease by just configuring a port into the appropriate VLAN.

The following topology shows a network with all hosts inside the same VLAN:

A network without VLANs

Without VLANs, a broadcast sent from host A would reach all devices on the network. Each device will receive and process broadcast frames, increasing the CPU overhead on each device and reducing the overall security of the network.

By placing interfaces on both switches into a separate VLAN, a broadcast from host A would reach only devices inside the same VLAN, since each VLAN is a separate broadcast domain. Hosts in other VLANs will not even be aware that the communication took place. This is shown in the picture below:

A network with VLANs

To reach hosts in a different VLAN, a router is needed.

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